To process and use impression compounds in dentistry, the directions for use normally specify suitable times, how the impression compound is to be handled, and when it is to be removed from the patient's mouth.
However, the specified times for processing and the setting behavior of the compounds are subject to various influences in dental practice, such as the actual mouth temperature and room temperature, the mixing energy applied, or the time which was spent on the mixing.
It is conceivable to incorporate indicators into the polymerizable compounds, which indicators are released during the polymerization and indicate the progress of the reaction, for example by a change in the color intensity. Such an attempt is described in WO-96/00560.
A disadvantage of this is that the compounds described contain a further component which may have a negative effect on the desired properties. In clearly definable signal of the setting process and it additionally requires continuous visual monitoring.
DE 29 906 343 U1 attempts to solve the problem by making available a unit for dispensing multi-component compounds, which unit is equipped with a timer clock for specifying a time relevant for the processing.
The unit described has the disadvantage, however, that the mixed compound is again subject to said external influences and, therefore, no clear indication can be given of when the setting process begins.
Moreover, industrial measuring mixers for checking the consistency of cement/water mixtures are known which are used for preparing concrete (DE 199 03 753 C1).
An ancillary device for torque measurement for stirring units, in particular for small-scale fermenters, is described in DE 28 50 486 A1.
DE 39 19 534 A1 discloses a method and a device for preparing bone cement. A propeller is lowered into a vessel in which the bone cement is to be prepared. The preparation of the bone cement is process-controlled.
None of the devices known from the prior art is suitable for determining the change in Theological properties of comparatively quickly hardening compounds, in particular dental impression compounds, which have to be prepared in portions and made available at short intervals one after another.
It is therefore an object of the present invention to propose a device and a method which solve the problems discussed above and inform the user of the progress of the curing of hardening compounds, in particular of dental impression compounds.
This object is achieved by a device and a method as are described below.
The term “setting profile” generally describes the changes in rheological properties occurring during the hardening or setting of hardenable compounds. The determination of the end of the processing time is closely linked to this.
The expression “processing time” is to be understood as the time during which the mixed compound can still be used in the intended manner. In the case of dental impression compounds, this is the time up to which the impression compound can be relatively easily lifted from the article of which an impression is to be made, without appreciably impairing the accuracy of the impression. This is usually the point in time at which the compound changes from the pasty, plastic state to a tough and resilient, rubber-like state.
The expression “hardenable compounds” includes all compounds which, as a result of a polymerization reaction, for example a free-radical, cationic or anionic addition reaction and/or condensation reaction and/or cement reaction, can change from a viscous, flowable, optionally plastically deformable state to a permanently shaped, rigid state.
Compounds whose setting process can preferably be recorded with the device according to the invention usually have the following properties before the start of the setting process. They are pasty, highly viscous substances which rigidify over a period in the range of 0.1 to 60 minutes, preferably 1 to 8 minutes. Before the start of the setting process, such compounds have, for example, a viscosity of class 0 to 3 as determined by the DIN 4823 consistency test, measured with a diameter of less than 80 mm. Materials which may be mentioned as possible examples are silicones, polyethers, epoxy resins and polyurethanes.
The Shore hardness A of the compounds measured according to DIN 53505 fifteen minutes after the end of the processing time usually lies in the range of 20 to 110, preferably in the range of 30 to 80.
The invention is suitable in particular for recording the setting behavior of sealing compounds and dental compounds, preferably dental impression compounds, for example those based on polyethers, A and C silicones, alginates and/or polyether silicones.
“Rheological property” is to be understood as meaning all properties whose change can be recorded via a physical and/or chemical measurement method. These include, in particular, properties such as viscosity, compressive strength, pH, conductivity, dielectric constant, impedance, capacitance, hardness, density and/or temperature.
In contrast to static mixers, the term “dynamic mixer” includes mixers which have at least one rotatably mounted part which is driven via a mixer shaft. Such mixers are described, for example, in DE 90 17 323 U1 or WO-A-98/43727. These mixers are what are referred to as continuous-flow mixers. Mixed material or paste is dispensed from the mixer during the mixing, whereas, during the measurement, the at least partially mixed material remains in the mixer and is not conveyed any further or dispensed.
A dynamic mixer generally comprises the following component parts: a housing, inlet openings, an outlet opening, and a rotatably mounted mixing element with mixing vanes. It is driven either centrally via a rotor which has mixing vanes located on it and rotates in the housing, or externally via a part of the housing serving as rotor and an inner body serving as stator, as is described in DE 19 947 331 A1.
Since mixers of this kind are usually disposable articles whose value is generally considered slight when compared to the substances which are to be mixed, these mixers have a relatively small mixing volume in order to keep the amount of mixed compound to be discarded as small as possible. Such disposable articles are usually made of plastic.
A sensor unit within the meaning of the invention is a unit which is able to record any change in state of the impression compound via a coupling to the mixer shaft. This includes capacitors, ultrasound sensors, pH electrodes, torque transducers, quartz oscillators, thermoelements, current meters, voltage meters, resistance meters, wire strain gages and force transducers.
The term “display unit” includes all units which are able to inform the person using the device of a change in the state of the impression compound, preferably in optical and/or acoustic form. This includes displays, in particular LED displays, and loudspeakers.
The terms “comprise” and “contain” introduce a nonexhaustive list of features. The fact that the word “a” is used before the mention of a feature in the claims does not exclude the possibility that said features can be present more than once, in the sense of “at least one”.
The invention has the following advantages:
The device according to the invention and the method according to the invention permit immediate monitoring of the process of hardening of the mixed compound, substantially independently of external influences.
In contrast to measurement mixers, which are known from the prior art, the change in rheological properties is not determined on the basis of a material sample which is subsequently further used, but instead on the basis of a material sample which remains in the mixer and is discarded together with the latter.
The measurement can be carried out directly in the device normally used for mixing and dispensing the hardening compounds. Arranging additional auxiliary means, such as a bypass, chambers, volumetric flask, measurement drives or probes, on known measurement devices is not necessary.
If the measurement takes place from the time at which no further mixed compound is dispensed through the mixer, the invention makes it possible to record the hardening process of the last-mixed compound in the mixer itself, which is usually discarded after the compound has hardened. The information as to when the last-mixed compound begins to harden is, in particular, of not inconsiderable importance to the user, for example the dentist, in preparing an impression of the hard dental tissue using impression compounds.
This represents a decisive advance over the method known from DE 299 06 343 U1, in which the time up to which the compound can be processed is predefined and thus not linked to the actual setting process.
The present invention thus means that dynamic mixers which are used to mix hardening compounds can be used, after the mixing process, to monitor the setting profile of the hardening compounds. The device is not limited to a certain type of mixer. Different mixers with different geometries can be used depending on the compounds which are to be mixed. A mixer can be changed very easily, and this can be done in less than one minute.
The compounds are usually prepared by mixing a base paste and a catalyzer paste. The speed of the setting process differs depending on the mixing ratio and on the substances. However, the device according to the invention is also suitable for determining the setting process of a mixture of three or more pastes or substances. In addition to paste/paste mixtures, the mixtures can also be paste/fluid mixtures or powder/fluid mixtures.
The pastes are mixed in a dynamic mixer, for example according to DE 90 17 323 U or WO-98/43727. This mixer is usually placed on a corresponding cartridge and driven by an electrically actuated mixing unit into which the cartridge can be fitted. Suitable mixing units are described in EP 0 492 413 A.
If the compound obtained by mixing is a dental impression compound, this is preferably used to fill a dental impression tray which is then fitted in a patient's mouth.
Since in this case the setting profile according to the invention is measured outside the mouth and the profile is temperature-dependent, the setting of the compound in the unit takes place somewhat more slowly than in the mouth. This ensures that in every case the compound has completely set before it is removed from the mouth.
The direct correlation between the setting profile in the mouth and in the mixing unit can be determined empirically as a function of the compound used.
The measurement of the change in a rheological property of the hardening compound and the comparison with a predeterminable threshold value are carried out using a measurement program which can be integrated into the control of the mixer shaft or is operated independently of the latter.
The measurement is preferably carried out directly via the mixer shaft of an electrically operated mixing unit, said mixer shaft driving a dynamic mixer. After an impression tray has been filled, the mixer shaft continues to be driven with the forward feed switched off, so that the torque applying on the shaft increases as the compactness of the compound increases. In this embodiment, the end of the processing time of the compound can be indicated by a visual or acoustic signal as soon as a determined threshold value is reached which can be individually set depending on the compound or is automatically set as soon as the cartridge containing the compound is placed in the mixing unit.
The mixer shaft is coupled, for example, to a torque transducer or a rotatable shaft which make it possible to determine the torque applied to the mixer vanes of the mixer or to the mixer shaft, which torque is proportional to the viscosity of the hardening compound. It is also possible to record the change in torque via the current consumption of the drive.
The change is preferably measured with the mixer shaft moving at a reduced speed of rotation compared to during mixing, in order not to unnecessarily introduce rotation energy into the hardening compound, which can lead to an undesirable increase in temperature. This additionally permits a more precise determination of the change. Speeds of rotation which can be used for the mixer shaft during the measurement process lie in the range of 100 to 103 and preferably in the range of 101 to 102 rpm.
In an alternative embodiment, the measurement process takes place via the mixer shaft in a pulsed mode or cyclically. For example, the measurement takes place only every 1, 5 or 10 seconds.
The measurement of the change in a rheological property of the hardening compound can also take place if the mixer shaft does not fully rotate. A turning of the mixer shaft in an angle range of 1 through 180°, preferably 10 through 90°, has proven advantageous.
However, another example of a suitable measuring unit is a quartz oscillator which generates a purely torsional oscillation, the latter being damped by the viscous properties of the hardening compound. The sensor head can be arranged in the form of a probe at or in the area of the tip of the mixer shaft and can be introduced into the compound via a seal in the area of the coupling of dynamic mixer/mixer shaft.
In contrast to measurement mixers known from the prior art, the present device not only allows a hardenable compound to be mixed and delivered, but also makes it possible, after the mixing and delivery process, to determine the setting profile of that part of the mixed compound remaining in the exchangeable, disposable mixer.
A preferred illustrative embodiment is explained below with reference to a FIGURE.